Covering the surface of a microscopic particle with a surface improving agent has been known as a technique to improve the particle surface. This technique is applied to either dry particles or particles in a liquid-phase. More specifically, this technique includes a kneading method, a stirring method using a medium, a spray dry method for coating a particle surface with a surface improving agent. These methods are well known and the detailed description is set forth in the following:
1. "Microscopic Particle Engineering: Basic and Application of Dispersion", pp. 123-136, edited by Japan Powder Industrial Technical Association and published by Asakura Shoten K. K.; and
2. "Today's Chemical Engineering 45 Microscopic Particle Engineering", edited by Chemical Engineering Association and published by Chemical Industrial Co., Ltd.
However, the above conventional particle surface improving methods have following problems:
(1) the particles are charged during the surface improving treatment, and the resulting charged particles are difficult to handle in the following procedure;
(2) the surface improving treatment takes quite a long time; and
(3) the surface improving treatment demands a complex manipulation and the actual operation depends on an expensive apparatus for the most part.